1. Field of the Invention
The present invention relates to a control valve that is suitable for controlling the discharging capacity of a variable displacement compressor for an automotive air conditioner.
2. Description of the Related Art
An automotive air conditioner generally includes a compressor, a condenser, an expander, an evaporator, and so forth. Here, the compressor discharges a high-temperature and high-pressure gas refrigerant produced by compressing a refrigerant flowing through a refrigeration cycle of a vehicle. The condenser condenses the gas refrigerant. The expander produces a low-temperature and low-pressure refrigerant by adiabatically expanding the condensed liquid refrigerant. The evaporator evaporates the refrigerant and thereby causes a heat exchange of the refrigerant with the air inside the vehicle. The refrigerant evaporated by the evaporator is again brought back to the compressor and thus circulates through the refrigeration cycle.
The compressor is, for example, a variable displacement compressor (hereinafter referred to simply as “compressor” also) capable of controlling the refrigerant discharging capacity in order to maintain a constant level of cooling capacity irrespective of the engine speed. This compressor has a piston for compression linked to a wobble plate that is mounted to a rotational shaft driven by an engine, and the compressor controls the refrigerant discharge rate by changing the stroke of the piston through changes in the angle of the wobble plate. The angle of the wobble plate can be changed continuously by changing the balance of pressure working on both faces of the piston as part of the discharged refrigerant is introduced into an airtight crankcase. The pressure within this crankcase (hereinafter referred to as “crank pressure”) Pc is controlled by a control valve for a variable displacement compressor (hereinafter referred to simply as “control valve” also), which is provided between the discharge chamber of the compressor and the crankcase or between the crankcase and the suction chamber thereof.
One of these control valves, such as one disclosed in Reference (1) in the following Related Art List, controls the crank pressure Pc through control of the amount of refrigerant introduced into the crankcase in accordance with a suction pressure Ps. This control valve includes a pressure-sensing section to develop a displacement by sensing the suction pressure Ps, a valve section for opening and closing the passage from the discharge chamber to the crankcase in response to a drive force from the pressure-sensing section, and a solenoid capable of changing the setting of the drive force from the pressure-sensing section by external electric current. The control valve like this opens and closes the valve section in such a manner as to maintain the suction pressure Ps at a pressure set by the external electric current. Generally, the suction pressure Ps is proportional to a refrigerant temperature at the exit of the evaporator, and thus the freezing or the like of the evaporator can be prevented by maintaining the pressure setting at or above a predetermined value. Also, when the engine load of a vehicle is high, the compressor can be operated at the minimum capacity by fully opening the valve section with the solenoid turned off and by setting the wobble plate substantially at a right angle to the rotational shaft with the crank pressure Pc set high.